I like to use Paypal for creating shipping labels, even if I'm shipping something that wasn't purchased on eBay or paid for through PayPal. It keeps everything in one place for me, the rates are often better, and it's easy to check up on delivery, etc.
However, if I don't have a balance sitting in my paypal account, it won't let me do it. At least I can't find an option to have it take payment directly out of my checking account (even though I have that option in virtually every other situation).
BUT -- if I instead go to the UPS site directly, create the shipment, and then select PayPal as the method of payment .... then I can have the money take out of my checking account if I don't have a balance.
If UPS is OK with it, why isn't PayPal OK with it?